Conventional field effect transistor devices (hereinafter, referred to as FET devices) use inorganic semiconductors such as silicon or germanium and require several stages of processes requiring much production cost such as photolithography and vacuum vapor deposition for forming circuit patterns. In semiconductor industries employing such production processes, requests for the reduction in production cost or large-area production in display are increased. However, it is difficult to realize the reduction in production cost or the enlargement of area in the inorganic semiconductor due to the constraints of manufacturing facilities. Further, since the process for forming a film of inorganic semiconductor such as silicon is operated at extremely high temperature, there has been a problem that the variety of materials capable of being used as a substrate are limited.
Therefore, organic field effect transistor devices, in which an organic semiconductor having excellent formability is used as a semiconductor layer, are proposed. By using the organic semiconductor as ink, it becomes possible to form circuit patterns directly on a substrate by ink-jet technology or screening technology.
Important parameters of the FET device performance include mobility and an on/off ratio. An improvement in mobility means that the on-current is increased. On the other hand, an improvement in on/off ratio means that the on-current is increased and the off-current is decreased. Both these improvements mean that a switching characteristic of the FET device is improved, and for example, in liquid crystal displays, this leads to realize high gradation. For example, in the case of liquid crystal display, mobility of 0.1 cm2/V·sec or more and an on/off ratio of 105 or more are required.
As organic semiconductors used for FET devices, low molecular organic semiconductors such as acene-based compounds including pentacene and tetracene, and metal phthalocyanine compounds are disclosed. However, for the low molecular organic semiconductors, a vacuum process such as vapor deposition is often employed and therefore there has been a problem that it is difficult to enlarge the substrate size and reduce in costs. Therefore, polymer organic semiconductors such as conjugated polymers and polythiophenes are disclosed. However, in conjugated polymers typified by poly(p-phenylene vinylene), it is difficult to obtain high orientation and adequate mobility has not been achieved. On the other hand, with respect to polythiophenes typified by poly(3-hexylthiophene) (P3HT), a great deal of research has been done because they are soluble in organic solvents and films can be formed from the solution by ink-jet technology or screening technology. However, there has been a problem that most of the polythiophenes are oxidatively doped with ambient oxygen to cause the increase in off-current and the decrease in on/off ratio.
In recent years, organic semiconductor materials containing soluble oligothiophenes are disclosed (for example, Patent Document 1 and Patent Document 2). However, in the FET devices using these materials, adequate mobility has not been achieved.
Further, as a technique for enhancing the mobility, a method of using a composite in which carbon nanotubes are dispersed in a film of an polymer organic semiconductor such as polythiophenes (for example, Patent Document 3), and a method of using a solid composition in which carbon nanorods or nanotubes are dispersed in organic semiconductor molecules (for example, Patent Document 4) are disclosed, but in all these methods, adequate mobility has not been achieved.    Patent Document 1: JP 2006-128601A (CLAIMS)    Patent Document 2: JP 2006-24908A (CLAIMS)    Patent Document 3: JP 2006-265534A (CLAIMS)    Patent Document 4: JP 2006-93699A (CLAIMS)